Hydrometallurgical processing of lateritic ores for nickel and cobalt recovery, based on high-pressure sulfuric acid leaching (HPAL), has been gaining technological and commercial acceptance over the last 20 years.
Once nickel and cobalt values are transferred from the laterite ore into the sulfuric acid leach solution in the HPAL autoclave, a number of different refining routes have been developed for recovering the nickel and cobalt from the leach solution into saleable products. Solvent extraction (SX), employing metal extractants dissolved in suitable hydrocarbon diluents, is often used in one or more steps within these refining routes.
One of the extractants used in the hydrometallurgical processing of lateritic ores for nickel and cobalt recovery is Cyanex 301, a solvent extractant developed by Cytec Industries (EP 021387). The use of Cyanex 301 allows for the selective transfer of nickel and cobalt from the sulfate leach solution into an upgraded hydrochloric acid leach solution, from which nickel and cobalt are separated and refined to final products (U.S. Pat. No. 5,378,262).
Bis(2,4,4-trimethylpentyl)dithiophosphinic acid (referred hereto as dithiophosphinic Acid or DTPA) is the active ingredient of the Cyanex 301. Being an organic thiol ([C8H17]2P(S)SH), DTPA can undergo oxidation. A common oxidation path generates disulfide:

The disulfide can be regenerated back to DTPA and Vale has developed regeneration processes to achieve that (U.S. Pat. No. 5,759,512 and U.S. Pat. No. 6,022,991). These regeneration processes support the commercial viability of using Cyanex 301 for metal extraction.
The oxidation of DTPA can result in the formation, over time, of oxidation products other than disulfide. These oxidation products can be generally described as organophosphorus compounds that contain sulfur and/or oxygen. These compounds do not have the extractive and metal selectivity properties of DTPA and cannot be readily reduced back to DTPA. Hence, as Cyanex 301 (DTPA) is added in order to maintain the metallurgy-required DTPA concentration in the organic solution, these compounds will accumulate over time.
Their presence in the organic solution can be followed by suitable methods of analysis, including phosphorus nuclear magnetic resonance (31P-NMR), chemical analysis for phosphorus and sulfur as well as acid titration determination for acids such as DTPA.
The accumulation of these compounds in the SX organic solution can impact the desirable metallurgy of the Cyanex 301 based SX system by impacting the physical properties of the organic solution, such as viscosity, interfacial tension and the like and/or by affecting the ability of the extractant system to maintain the desired degree of metal extraction selectivity.
Thus, removal for some of the SX organic solution (organic bleed) from the operating organic circuit becomes necessary. The obvious disadvantage of this approach is the loss of DTPA with the organic bleed. Furthermore, disposal of the organic bleed itself will add further costs.
Therefore, a process that would enable the recovery of DTPA from the organic bleed and its separation from the accumulating oxidation products would be metallurgically and operationally advantageous as well as economically desirable.
Given that the DTPA content of Cyanex 301 is about 75-80% [Cyanex® 301 Extractant—Technical brochure, Cytec] and that it also contains other organothiophosphorus compounds, a purification technique was developed by Zhu et al [Zhu, Y., Chen, J., Jiao R., 1996, Extraction of Am(III) and Eu(III) from Nitrate Solution with Purified Cyanex 301, Solvent Extraction and Ion Exchange, 14(1), 61-68] aimed at isolating high purity DTPA, primarily for the purpose of fundamental metal extraction chemistry research.
In the Zhu purification technique, Cyanex 301 is contacted with ammonium carbonate solution at 70° C. for one hour to produce the ammonium salt of DTPA (ammonium dithiophosphinate) and then the mixture is left overnight at 0° C. to allow for the crystallization of the ammonium salt; the crystalized solids are eventually converted back to DTPA by contact with 4 N hydrochloric acid. This purification technique has been subsequently used by other researchers, e.g., [Groenewold, G. S, Peterman, P. R., Klaehn, J. R., Delmau, L. H., Marc P., Custelcean, R., 2012, Oxidative degradation of bis(2,4,4-trimethylpentyl)dithiophosphinic acid in nitric acid studied by electrospray ionization mass spectrometry, Rapid Commun. Mass Spectrom., 26, 2195-2203.] for similar purposes.
Applying this technique on an industrial scale will be very difficult and costly for the following reasons:                use of elevated temperatures and prolonged time for the contact with the ammonium carbonate solution, which could result in further oxidation of DTPA;        need to cool the mixture to 0° C. for a long period of time in order to produce the ammonium dithiophosphinate solids; and        need to carry out filtration of the ammonium dithiophosphinate solids from the organic solution, while likely having to maintain temperatures near 0° C.        
Therefore, there is a need for a method that will be able to effectively separate DTPA from other organophosphorus impurities while avoiding having to operate at extreme temperatures or producing, and having to deal with, organic solids.
It has now been discovered that DTPA can be effectively and efficiently recovered from DTPA-containing organic solutions and various other organophosphorus compounds, as described earlier, in which solutions DTPA is present in its acid form and/or in its base metals-salt form, where the group of base metals include nickel, cobalt, zinc, copper, chromium and the like, without the formation of solids or the necessity of maintaining low temperatures over long period of time.
The method avoids the need for elevated temperatures (above 70° C.), the need for operating at very low (near 0° C.) temperatures, the prolonged reaction times and the generation and handling of organic solids.
This is accomplished by contacting the DTPA-containing organic solution with a suitable aqueous base solution supplied in excess of the stoichiometric requirement for the acid-base neutralization or other metal cation-exchange reactions for the contained DTPA. This prevents the formation of solids that would otherwise form from the treated DTPA-containing solution.